Session Q41: Focus Session: Fundamental Issues in Interfacial Charge Transport for Energy Applications II

First principles simulations of materials and processes in photo- and electro-catalysis

I shall discuss applications of electronic structure calculations and molecular dynamics simulations to understand materials properties and reaction mechanisms in photo- and electro-catalysis. Examples will include studies of the interface between water and titanium dioxide (TiO$_{2})$, a widely used photocatalyst capable of splitting water in O$_{2}$ + H$_{2}$, and the cycle of H$_{2}$ production from water by the active site of an enzyme of hydrogen-producing bacteria, the di-iron hydrogenase, linked to a pyrite electrode.   

Theoretical studies on a new pattern of laser-driven systems: towards elucidation of direct photo-injection in dye-sensitized solar cells

We theoretically and numerically investigated a new type of analytically solvable laser-driven systems inspired by electron-injection dynamics in dye-sensitized solar cells. The simple analytical expressions were found to be useful for understanding the difference between dye excitation and direct photo-injection occurring between dye molecule and semiconductor nanoparticles. More importantly, we propose a method for discriminating experimentally dye excitation and direct photo-injection by using time-dependent fluorescence. We found that dye excitation shows no significant quantum beat whereas the direct photo-injection shows a significant quantum beat.   

Investigation of the Potential Difference between C60 and TiOPc on Ag(111) by Local Probe Techniques

One challenge for increasing efficiency of organic photovoltaics is to understand the barrier to exciton separation that exists at the interface between organic molecules. Here we report a local probe measurement of the potential barrier at the interface between submonolayer C60, a good electron acceptor, and honeycomb phase TiOPc, an organic with high hole mobility, on Ag(111). We employ UHV AFM (atomic force microscopy) and KPFM (Kelvin probe force microscopy) to obtain simultaneous images of the potential and topographic landscapes. This technique allows for high spatial resolution of both the potential and the topography. In addition to reporting the work function difference between C60 and TiOPc, we investigate the work function for C60 on Ag(111).   

Ultrafast proton coupled charge transfer dynamics in photocatalysis

In this talk I will present our experimental and theoretical studies on the nature of electron and hole acceptor states and their dynamics for protic solvent molecule (H$_{2}$O, CH$_{3}$OH) covered TiO$_{2}$ surfaces. Electron-hole pair generation by band gap excitation can introduce charges into protic solvent/TiO$_{2}$ interface, which can drive photocatalytic processes. By time resolved two-photon photoemission and DFT electronic structure calculations we identify the partially solvated or ``wet'' electron accepter states, and their proton-coupled electron transfer (PCET) dynamics. Because holes are through to be the primary reagents for photocatalysis on TiO$_{2}$, we also explore possible hole driven PCET dynamic pathways.   

First-principles study on Ru(4,4',4''-tricarboxy-2,2':6',2''-terpyridine)(NCS)$_3$ sensitizer on TiO$_2$ anatase(101) surface: Adsorbed structures and electronic states for dye-sensitized solar cells

Dye-sensitized solar cells are expected as a cost effective solar-to-electricity energy conversion devices. The efficiency of the power conversion is greater than 10\% when Ru(II) polypyridyl sensitizers are used. For further improvement of the efficiency, we need to understand the adsorbed structures at atomistic level in detail. In this study, we investigated the adsorbed structures of Ru(4,4',4''-tricarboxy-2,2':6',2''-terpyridine)(NCS)$_3$ sensitizer on TiO$_2$ anatase(101) surface. For four possible adsorbed structures (two candidates have one adsorbed carboxyl group(one-leg) and the others have two adsorbed groups(two-leg)), we found the adsorption energies are quite similar within 0.4 eV. This is attributed to the presence of the hydrogen bond between the hydrogen of carboxyl group and the oxygen of the surface in the one-leg structure. We also calculated the excited states of the four structures of the sensitizer by TDDFT and found that the UV spectrum shift depending on the structure differences.   

Reverse-engineering the atomic-scale structure of the TiO2/N3 interface in dye-sensitized solar cells using O1s core-level shifts

Dye-sensitized solar cells employing mesoporous titania films sensitized with ruthenium-based dyes have shown consistently good performance over the past two decades. Understanding the process of charge injection in these devices requires accurate atomistic models of the interface between the light-absorbing dye and the semiconducting substrate. Despite considerable efforts devoted to the experimental and theoretical investigation of such interfaces, their atomistic nature remains controversial. In this work we pursue a novel computational approach to the study of the semiconductor/dye interface which does not rely on the calculated adsorption energies. In our approach we reverse-engineer photoemission data through the first-principles calculation of O1s core-level spectra for a number of candidate interface models. Our calculations allow us to discard some of the adsorption geometries previously proposed and point to an interface model which reconciles conflicting assignments based either on photoemission or infrared data.   

Studies of Interfacial Electronic Processes in Nanoporous TiO2 Thin-Films

Metal-oxide nanoparticles sensitized to visible light by covalent attachment of molecular adsorbates have attracted considerable attention in recent years due their central role in technologies for solar energy conversion, including dye-sensitized solar cells (DSSCs) and solar photocatalysis. However, the mechanisms of interfacial electron transfer and subsequent electron transport induced by photoexcitation of the molecular adsorbates remain only partially understood. We report recent progress in studies of nanoporous TiO2 thin-films functionalized with molecular adsorbates, with emphasis on interfacial electron injection, molecular rectification and the mechanism of electron transport through sintered TiO2 nanoparticles in thin-films relevant to DSSCs.   

Efficient adsorbate transport by electron wind: The role of resonant photoexcitation

We study the surface electromigration force acting on an organic molecule at a conducting (metal) surface. The dominant contribution to the force comes from the scattering of metallic electrons off the molecule, as they tunnel to and from nearby molecular orbitals. When metal carries non-zero current, the net force is directed with the current flow. This force, however, is often too small for efficient transport of adsorbed molecules and only reveals itself through a contribution to the metal resistivity. We show that surface-molecule electron wind force can be substantially enhanced and controlled by exploiting appropriate resonances between molecular and metallic states activated by coherent light. This effect opens a path to new surface-molecule functionality, including high resolution spatially controlled force patterns, controlled molecule motion, etc.   

Real-time observation of bond-by-bond interface formation during oxidation of H-terminated (111)Si by second-harmonic generation

Structure of solids is typically determined at the atomic level by techniques such as X-ray and electron diffraction, which are sensitive to positions of atomic nuclei. However, structure is determined by bonds between atoms, which are difficult to measure directly. We combine second-harmonic generation and the bond-charge model of nonlinear optics to probe, in real time, the dynamics of bond-by-bond chemical changes during the oxidation of H-terminated (111)Si, a surface that has been well characterized by static methods. Oxidation is activated by applied macroscopic strain, and exhibits anisotropic kinetics with one of the three equivalent back- bonds of on-axis samples reacting differently from the other two. This also leads to transient changes in bond directions.~Strain is known to increase oxidation rate of Si for thermal oxides, however its affects at the microscopic scale has not been studied at the bond level. By comparing results for surfaces strained in different directions, we show that in-plane control of surface chemistry is possible. The use of nonlinear optics as a bond-specific characterization tool is readily adaptable for studying structural and chemical dynamics in many other condensed-matter systems.